David Cameron owes it all to Ken Clarke. Although it’s pretty certain he doesn’t see it that way, there is no doubt that Cameron won the Tory party leadership because the support of Tory centrists seeped to away from Clarke about half way through the contest.
The reasons for Clarke’s failure were clear enough at the time. He was complacent and underestimated the hunger of the Cameron camp; his outside interests, especially in the tobacco industry, were embarrassing to the party and meant that he spent relatively little time in the Commons.